KA-relevant question

[basophilic]

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A species of cactus is introduced to a region at time 0. Fast forward a million years and you find the cactus has a bimodal distribution in terms of needle length: highest populations are of the shortest and longest needle lengths but very few intermediate lengths. So at t = million yrs, you'd say it has undergone disruptive selection. So strictly in terms of terminology, would you say at t = 0 that it is undergoing "disruptive selection" or is it undergoing "natural selection"?

 

[MrRed]

I would think -- and this is only my opinion -- that it would be more appropriate to say it is undergoing natural selection.
Reason being: disruptive selection is specifically selection against the mean; the intermediate phenotype. How can you qualify/quantify this? The ONLY way to actually affirm that there is disruptive selection present would be the observation that the majority of phenotypes for the trait being inspected present at the two extremes of the spectrum -- in this case only the longest or shortest of needles.
If you look at the cactus at t=0; there will be absolutely no way to infer that disruptive selection is occurring; however you can definitely assume that natural selection will run its course -- as it has for arguably every species in the history of our Earth.

 

[bee17]

You can't tell if any selection is happening at t=0, as evolution is change in allele frequency over time. If no time has passed, you can't evaluate selection or evolution. It's a moot point anyway, as disruptive selection is a type of natural selection. If a population is undergoing disruptive selection, then it is also undergoing natural selection.